ion channels

Ion ChannelsJohn Koester

jdk3

References:

•Kandel, Schwartz and Jessell (2000): Principles of Neural Science, 4th edition, chapter 5

•Hille, B. (2001) Ion Channels of Excitable Membranes, 3rd edition

Outline Why ion channels? Channel structure Ion channels have three basic functional properties

Conduct Select Gate

Evolutionary relationships between ion channels Various factors contribute to ion channel diversity

Ions Cannot Diffuse Across the Hydrophobic Barrier of the Lipid Bilayer

Ion Channels Provide a Polar Environment for Diffusion of Ions Across the Membrane

Specialized Functions of Ion Channels

Mediate the generation, conduction and transmission of electrical signals in the nervous system

Control the release of neurotransmitters and hormones

Initiate muscle contraction

Transfer small molecules between cells (gap junctions)

Mediate fluid transport in secretory cells

Control motility of growing and migrating cells

Provide selective permeability properties important for various intracellular organelles

Outline Why ion channels? Channel structure Ion channels have three basic functional properties

Conduct Select Gate

Evolutionary relationships between ion channels Various factors contribute to ion channel diversity

Channels are Made Up of Subunits

Outline Why ion channels? Channel structure Ion channels have three basic functional properties

Conduct Select Gate

Evolutionary relationships between ion channels Various factors contribute to ion channel diversity

•Ion Channels Act As Catalysts

•Speed up fluxes

•Do not impart energy

•Driving force is provided by electrochemical potential

•Ion Channels Conduct Up to 108 Ions/sec

Conduction

Unlike Channels, Ion Pumps Do Not Provide a Continuous Pathway Through the Membrane

Na+

K+

Outline Why ion channels? Channel structure Ion channels have three basic functional properties

Conduct Select Gate

Evolutionary relationships between ion channels Various factors contribute to ion channel diversity

Ion Channels are Selectively Permeable

Cation PermeableNa+

K+

Ca++

Na+, Ca++, K+

Anion PermeableCl -

Structure of K+ Channel HasMultiple Functional Adaptations

SelectivityFilter

Outline Why ion channels? Channel structure Ion channels have three basic functional properties

Conduct Select Gate

Evolutionary relationships between ion channels Various factors contribute to ion channel diversity

Single Channel Openings are All-or-None in Amplitude,With Stochastically Distributed Open and Closed Times

Closed

Open

2 pA

20 msec

There are Two Major Types of Gating Actions

Gating Can Involve Conformational Changes Along the Channel Walls

Gating Can Involve Plugging the Channel

Gating Can Result from Plugging by Cytoplasmic or Extracellular Gating Particles

There are Five Types ofGating Controls

1) Ligand Binding

Extracellular

Cytoplasmic

2) Phosphorylation

3) Voltage-gated

4) Mechanical Force-Gated

Stretch

Change Membrane Potential

5) Temperature-gated

Cur

rent

Temperature (º C.)

20 25 30 35 40 45 50

Heat-SensitiveCold-Sensitive

Modifiers of Channel Gating

Binding of Exogenous Ligands Can Block Gating

(ACh)

(Curare)

(BTx)

Ion Permeation Can be Prevented by Pore Blockers

PCP

Glutamate-Activated Channel

Exogenous Modulators Can Modify the Action of Endogenous Regulators

Open

Closed

Open

Closed

Current

Time

Outline Why ion channels? Ion channels have three basic functional properties

Conduct Select Gate

Evolutionary relationships between ion channels Various factors contribute to ion channel diversity

Evolution Operates More Like a TinkererThan an Engineer

Ion Channel Gene Superfamilies

I) Channels Activated by Neurotransmitter-Binding(pentameric channel structure):

•Acetylcholine•GABA•Glycine•Serotonin

II) Channels Activated by ATP or Purine Nucleotide-Binding (quatrameric or trimeric channel structure)

Ion Channel Gene Superfamilies

III) Channels With Quatrameric Structure Related to Voltage-Gated, Cation-Permeant Channels: A) Voltage-gated:

•K+ permeant•Na+ permeant•Ca++ permeant•Cation non-specific-permeant

B) Cyclic Nucleotide-Gated (Cation non-specific-permeant)

C) TRP Family (Cation Non-specific); Gated by: • osmolarity

• pH• mechanical force (hearing, etc.)• ligand binding• temperature

D) Channels Activated by Glutamate-Binding •quatrameric channel structure

•cation non-specific permeability

Ion Channel Gene Superfamilies

IV) “CLC” Family of Cl--Permeant Channels (dimeric structure):

Gated by:•Voltage•Cell Swelling •pH

V) Gap Junction Channels (non-specific permeability; hexameric structure)

Outline Why ion channels? Channel structure Ion channels have three basic functional properties

Conduct Select Gate

Evolutionary relationships between ion channels Various factors contribute to ion channel diversity

Different Genes Encode Different Pore-Forming Subunits

Different Pore-Forming Subunits Combine in Various Combinations

The Same Pore-Forming Subunits CanCombine with Different Accessory Subunits

Alternative Splicing of Pre-mRNA

Post-Transcriptional Editing of pre-mRNA

Generator Potentials, Synaptic Potentials and Action Potentials All Can Be Described by the Equivalent

Circuit Model of the Membrane

PNS, Fig 2-11

The Nerve (or Muscle) Cell can be Represented by aCollection of Batteries, Resistors and Capacitors

Equivalent Circuit Model of the Neuron

+ + + +

- - - -

Vm = Q/C

∆Vm = ∆Q/C

The Lipid Bilayer Acts Like a Capacitor

∆Q must change before∆Vm can change

Change in Charge Separation AcrossMembrane Capacitance is Required to Change

Membrane Potential

--

----

--

--

--

--

------ --

--

--

--+

+

+

+

+

+

+

+

--

--

+

+

+

+

+

+

+

+

The Bulk Solution Remains Electroneutral

PNS, Fig 7-1

Each K+ Channel Acts as a Conductor (Resistance)

PNS, Fig 7-5

Ion Channel Selectivity and Ionic Concentration Gradient Result in an Electromotive Force

PNS, Fig 7-3

An Ion Channel Acts Both as a Conductor and as a Battery

RT [K+]o

zF [K+]i

•lnEK =

PNS, Fig 7-6

An Ionic Battery Contributes to VM in Proportion to the

Membrane Conductance for that Ion

Experimental Set-up forInjecting Current into a Neuron

PNS, Fig 7-2

Because of Membrane Capacitance,Voltage Always Lags Current Flow

Rin x Cin

PNS, Fig 8-3

Length Constant = √rm/ra

PNS, Fig 8-5